Throughfall exclusion and fertilization effects on tropical dry forest tree plantations, a large-scale experiment

Abstract

Across tropical ecosystems, global environmen- tal change is causing drier climatic conditions and increased nutrient deposition. Such changes represent large uncertain- ties due to unknown interactions between drought and nutri- ent availability in controlling ecosystem net primary produc- tivity (NPP). Using a large-scale manipulative experiment, we studied for 4 years whether nutrient availability affects the individual and integrated responses of aboveground and belowground ecosystem processes to throughfall exclusion in 30-year-old mixed plantations of tropical dry forest tree species in Guanacaste, Costa Rica. We used a factorial design with four treatments: control, fertilization (F), drought (D), and drought +fertilization (D +F). While we found that a 13 %–15 % reduction in soil moisture only led to weak ef- fects in the studied ecosystem processes, NPP increased as a function of F and D +F. The relative contribution of each biomass flux to NPP varied depending on the treatment, with woody biomass being more important for F and root biomass for D +F and D. Moreover, the F treatment showed modest increases in maximum canopy cover. Plant functional type (i.e., N fixation or deciduousness) and not the experimen- tal manipulations was the main source of variation in tree growth. Belowground processes also responded to experi- mental treatments, as we found a decrease in nodulation for F plots and an increase in microbial carbon use efficiency for F and D plots. Our results emphasize that nutrient avail- ability, more so than modest reductions in soil moisture, lim- its ecosystem processes in tropical dry forests and that soil fertility interactions with other aspects of drought intensity (e.g., vapor pressure deficit) are yet to be explored.